Process for bleaching wood pulp with fluorine,hydrofluoric acid,and oxygen difluoride



F'IPSSQ? United States Patent Int. (:1. D21 9/12 US. Cl. 162-63 ClalmSABSTRACT OF THE DISCLOSURE A method of bleaching wood pulp withfluorine, hydrofluoric acid, or oxygen difluoride.v The plup iscontacted This invention relates to the process of bleaching wood pulpand natural and synthetic fibers, such as are used the paper makingindustry, the preparation of cloth fabrics and films. In one particularembodiment, it relates to improvement in the method of bleaching woodpulp used in the preparation of paper to give an improved bleachedproduct.

The bleaching of wood pulp and other cellulosic fibers to a highbrightness with yaryingbleaching agents such as chlorine or hypochloriteis frequently accompanied by degradation during bleaching. In spite ofthe large amount of work that has been done in the field, and the largenumber of 'multi-stage bleaching processes that have been developed,there is no completely satisfactory answer to the problem of bleachingwithout degradation. Chlorine dioxide does have the property ofbleaching without substantial degradation, but it is quite costly andrequires specialhandling due to its explosive tendency.

The principal object of my invention is to provide a process for thebleaching of fibrous materials through chemical action by employingfluorine either alone or as its compounds. Another object of myinvention is to provide improved bleached brightness over the use ofconventional bleaching chemicals. Another object of my invention is toprovide improved bleaching with lesser reduction in strength, greaterpermanence and ease of handling than with the use of conventionalbleaching chemicals. A further object of my invention is to replace orsupplement the currently employed bleaching chemicals by the use offluorine or fluorine compounds to attain an improved quality and/ oreconomy.

I have discovered that the reaction product of various fluorinecompounds produces an effective bleaching agent. Thus, I have discoveredthat a very substantial increase in brightness can be secured in achemical wood pulp by treating it with fluorine compounds or gaseousfluorine and that the increase in brightness is accomplished by verylittle degradation in the strength of the cellulose.

"The dispersing viscosity and the bursting and tear strength of the pulpare not changed appreciably by bleaching using my novel process, whereaswhen chlorine or hypochlorites are employed, there is usually asubstantial loss in strength. The treatment can be carried out using theconventional mechanical procedures and equipment'for bleaching withchlorine, or chlorine compounds.

Current bleaching technology, particularly applied to the bleaching ofchemical wood pulp for paper, uses chlorine, caustic soda, calcium andsodium hypochlorites, chlorine dioxide, sodium and hydrogen peroxide.With ground wood pulp, reducing agents, such as zinc or 3,481,826Patented Dec. 2, I969 "ice sodium hydrosulphite may be used, with orwithoutthe use of hydrogen peroxide. In the usual conventionalprocesses, several chemicals are used in series or stages, with residuesfrom the reactions being washed out between the stages. A conventionalbleaching sequence, for example, uses a series of stages that have beendesignated by certain letters. For example, C is used to designatechlorination, E for extraction using caustic soda, H for sodium orcalcium hypochloride, D for chlorine dioxide and P for hydrogenperoxide. Thus, for example, a

bleaching sequence for a sulphite pulp might be designated as CEH; C forchlorination, E for caustic soda extraction, and H for hypochloritefiAbleaching cycle for sulphate pulp might be designated as C EI IED; i.e.chlorination, extraction, hypochlorite, extraction and chlorine dioxide.Another cycle might be six stage as CEHEHD; i.e. chlorination,extraction, hypochlorite, extraction, hypochlorite and chlorine dioxide.Another cycle might be CEDED. It is apparent that many othercombinations may be used according to the properties desired in thebleached pulp and the starting material used in the process. v

The C stage is used mostly for purification of the pulp by chlorinatingthe non-cellulosic materials present such as lignin. The E stage is usedto solubilize and washout chlorinated substances. The H stage is used tobleach and white the pulp and the D stage is added to further whiten thepulp without decreasing its strength. The P stage is sometimes used atthe end to reduce loss of brightness of the pulp upon aging.

In each stage it is important to control the concentration orconsistency of the pulp, the percentage of the chemical, thetemperature, the contact time, sometimes known as dwell time, and thepH. Typical chlorine bleaching systems and the modified system of myinvention are set out in the examples below.

In the first stage of my process, I use a fluorine gas previouslydissolved in water or passed directly into the pulp suspension. Thegaseous fluorine may be generated in the presence of the pulp. Anothersource of fluorine is the anhydrous hydrogen fluoride of commerce whichmay be added directly to the pulp suspension alone or in combinationwith gaseous fluorine or sodium fluoride. The effects of the fluorine orits compounds upon the pulp are completed before the pulp is exposed tohuman contact. The fluorine bleaching treatment of my invention can becarried out by contacting the pulp with fluorine or a suitable fluorinecompound in an amount equal to 0.5 to 9 percent by weight based on theweight of the pulp at a temperature of about to F. for a period of 1 to4 hours. Suitable bleaching agents which can be used in any processinclude fluorine, hydrofluorine acid and oxygen difluoride.

I prefer to designate my fluorine stage as F so that it might be writtenin a bleaching sequence as FEH mean ing: fluorine, extraction andhypochlorite. For a sulphate pulp, it might be a fluorine stage in placeof the chlorine stage.

Particularly useful forms of fluorine compounds are the oxy compounds.For example, oxygen difluoride (OR) is prepared in a continuous processby bubbling flnorineinto a 2% solution of sodium hydroxide. The efiluentis a gas that can be passed directly into the pulp at any stage of theprocess, particularly as the final superbleaching stage usually carriedout by the use of chlorine dioxide. In some cases where chlorine dioxideis used in a stage other than the final stage, oxygen difluoride may beused to replace it in that stage. Oxygen difluoride is a particularlysuitable source of fluorine for the purpose of my invention since itdecomposes into the components oxygen and fluorine at the temperaturesof operation of the normal 3 bleaching system, and where it is used Iprefer to designate it as the stage.

The amounts of the fluorine compounds to be added in my novel processwould closely parallel the amounts where chlorine compounds are used.Thus, if fluorine is added, it is used in the amount of 4 to 9%, usuallyabout 7% on the pulp in the first stage, based on the gaseous fluorine,added as such or generated in the pulp. The pulp would normally bepreferably about 3% consistency. The reaction would be carried out atthe temperature of preferably about 80 to 90 F. and the retention timewould be from 1 to 4 hours, usually about 1% hours. The extraction stagewith caustic soda would be carried out with a solution of /2 to 3%caustic soda, preferably about 2 /2% caustic soda at a consistency ofpreferably about 12% and a temperature of about 120 F. to 180 F. for aperiod of preferably about 1 hour and an alkaline pH. In the dioxidestage of the process, the reaction would be carried out preferably withabout 0.5% of the fluorine compound at preferably about 12% consistency,at a temperature about 160 F. for a period of about 1 hour. In normaloperation, the pulp would be washed between stages. If the pulp were tobe treated with a peroxide, it would be a final treatment of thebleached pulp at about 1% usually without subsequent washing. Morebroadly this embodiment of my invention comprises the steps of:fiuorination of the pulp with fluorine or a suitable fluorine compound;washing and extracting the pulp with a sodium hydroxide solution;treating the pulp with oxygen difluoride; and then washing andrecovering the bleached product. A more specific form of this generalembodiment the fluorination is carried out using a gaseous hydrogenfluorine concentration of 2 to 9% andthe oxygen difluoride treatment iscarried out with .5 to 15% of the compound (i.e. oxygen difluoride) anda pulp consistency of 3 to 12%.

Considerable variation is practiced in the amount of chemicals,consistency of the pulp, temperature, dwell time and the number andsequence of steps as may be arranged to obtained the desired results asto pulp quality in brightness, opacity, strength, viscosity,permanganate number and the like, within the limits of the original pulpand the effectiveness of the chemical employed.

With ground wood pulps a single stage uses about 1% sodium or zinchydrosulphite at 2 to 5% consistency for 1 to 2 hours and 110 to 150 F.at pH 4.5 to 6 for the zinc and pH 6-7 for the sodium salt. It may besupplemented by a stage of 1% sodium or hydrogen peroxide (stabilizedwith sodium silicate and magnesium sulphate) at 10 to consistency andalkaline pH for 3 to 4 hours at 100 F.

My invention is further illustrated by the following specific butnon-limiting examples.

EXAMPLE I 100 grams of a southern pine sulphate pulp having a viscosityof 58.0 centipoises at 3% consistency is treated with 5% fluorine gas,added as such, at a temperature of 75 F. for a retention time of onehour. The pulp is then washed. The second stage is a treatment with 2.0%caustic soda for one hour at 12% consistency and 160 F. The pulp is thenwashed. The third stage is a treatment with 1.5% sodium hypochlorite for2 /2 hours at 12% consistency, pH 10.0 and 95 F. The pulp is thenwashed. The fourth stage is extraction using 1.0% caustic soda for 1hour at 12% consistency and 160 F. The pulp is then washed. The fifthstage is a treatment with 1% chlorine dioxide for 4 to 5 hours at 12%consistency and 160 F. The pulp is then given its final washing. Thebrightness is 90 and the viscosity is 28.0 centipoises.

In this example, oxydifluoride might be used to replace chlorine dioxidein the fifth stage.

4 EXAMPLE II grams of a soft wood sulphite pulp is bleached by the FEHsequence with washings between the stages and at the end. In the F stage3% fluorine is used at 3% consistency for /2 to 1 hour at 60 F. with apH of 2-3, and the E stage 0.5% castic soda is used at 8% consistencyfor one hour at F., and in the H stage 3% available chlorine is used at12% consistency for 2 /2 hours at 80 F. The pulp brightness is 87.

In making dissolving pulp, where a caustic soak stage is used, a finaltreatment with peroxide is precluded by the presence of silicates in theperoxide solution. In place of such a P stage and 0 stage mayv beemployed.

EXAMPLE III 100 grams of an aspen semi-chemical pulp cooked by the coldsoda process with a brightness of 39 is treated with 2% zinchydros'ulphite followed by 1% oxydifluoride to obtain a brightness of63.

EXAMPLE IV 100 grams of a ground wood pulp with a brightness of 58 istreated with 1% oxydifluoride to obtain a brightness of 65.

What is claimed is:

1. A process for bleaching wood pulp which comprises contacting the pulpwith a quantity of oxygen difluoride equal to about 0.5 to 9% by weight,based on the weight of pulp, for a sufficient time to bleach said pulp.

2. The process of claim 1 wherein said pulp'is contacted with saidoxygen difluoride for from 1 to 4 hours at a temperature of about 90 to160 F.

3. A process for bleaching wood pulp which comprises contacting the pulpwith a quantity, equal to about .5 to 9% by weight based on the weightof pulp, of a bleaching agent selected from the group consisting offluorine, hydrofluoric acid, and oxygen difluoride, for from 1 to 4hours at a temperature of about from 60 to 160 F., whereby said pulp isbleached, washing said bleached pulp and extracting said bleached pulpwith /2 to 3% caustic soda solution at a temperature of about to 180 F.,washing the extracted pulp and then treating the pulp with oxygendifluoride in an amount equal to about .5 to 15% by weight based on theweight of pulp.

4. The process of claim 3 wherein the first bleaching treatment iscarried out using gaseous hydrogen fluoride at a fluorine concentrationof 2 to 9% and the oxygen difluoride treatment is carried out with 0.5to 15 of the oxygen difluoride and with a pulp consistency of from 3 to12%.

.5. The process" of claim 3 wherein the oxygen difluoride treatment iscarried out by contacting the pulp with about .5% oxygen difluoride at atemperature of about F. for about 1 hour and using a pulp consistency ofabout 12%.

References Cited Kirk-Othmer, Encyclopedia of Chemical Technology 2nded., vol. 9., Interscience, N.Y. (1966), p. 527.

Clark, J. T., Determination of Lignin by Hydrofluoric Acid in Tappi,vol. 45, No. 4, April 1962, pp. 310-314.

Casey, I. P., Pulp and Paper, vol.'I, Interscience, New York (1960), p.520.

S. LEON BASHORE, Primary Examiner R. D. BAJEFSKY, Assistant Examiner US.Cl. X.R.

